Water softening systems are used in households and by industry to replace hardness cations such as calcium and magnesium with sodium ions by passing an incoming water supply through a bed of cationic exchange resin beads or particles. When the ion exchange resin bed periodically becomes saturated with ions removed from the incoming water, and depleted of sodium ions, it is recharged by passing a brine solution consisting essentially of sodium chloride through the resin bed. This replenishes the bed with sodium ions and removes the calcium, magnesium, or other ions previously removed from the incoming water.
The exchange capacity of an ion exchange resin bed deteriorates as impurities in the incoming water collect in the resin bed and are not removed by the recharging process. Sooner or later, depending on the level of maintenance of the resin bed and the characteristics of the water supply being softened, the resin becomes "fouled", meaning that the resin bed's capacity to soften water has diminished so much that the resin must be specially treated to restore its softening capacity.
The presence of iron in the incoming water supply is the most common cause of resin fouling. Iron can exist in the water supply as clear water iron, red water iron, bacterial iron, colloidal iron, or tannate iron.
Clear water iron is iron in the divalent (ferrous), soluble state. Clear water iron is not visible when the water is drawn, but when the water is allowed to stand for a prolonged period the ferrous ions are oxidized by air to become ferric or trivalent ions, which settle as a precipitate of ferric hydroxide. The iron may also oxidize after having been exchanged into a resin bed, which will prevent it from being removed by regeneration.
Red water iron is already oxidized to the ferric state when it reaches the water softener. Water containing red water iron is cloudy and orange when drawn. This form of iron may be filtered by the resin bed or may be passed and be present in the softened water.
Bacterial iron is a third troublesome form of iron, and is caused by iron crenothrix bacteria which feed on the iron in the water supply. These bacteria thrive in water softener resin beds supplied with ample iron, and the resulting biomass clouds the water system, creates a bad taste and odor in the softened water, and occasionally releases large, unsightly masses of rust colored material.
Colloidal iron is similar to red water iron, but is composed of particles too small to settle. Colloidal iron will normally pass directly through a water softener.
Finally, tannate iron, which is quite similar in appearance to colloidal iron, is ferric iron complexed and held in solution by tannates or other naturally occurring soil ingredients. This final form of iron usually passes through a water softening resin bed.
Iron present in any of the previously discussed forms can foul the resin bed. Oxidation of ferrous iron captured by the resin beads can crack them, thereby physically degrading the resin bed as well.
The iron problem is well known in the softening art, and attempts have been made to remove iron in all its forms from water softening resin beds. Other chelating compounds for sequestering iron are listed in column 2, lines 47 through 53 of U.S. Pat. No. 3,454,503, issued to Blankenhorn et al. on July 8, 1969. Among the iron chelating materials disclosed therein is citric acid. U.S. Pat. No. 2,769,787, issued to Diamond on Nov. 6, 1956, also discloses a method for regenerating cation exchange resins fouled by iron by adding to the brine regeneration medium any of a variety of organic acids, particularly citric acid. Citric acid is used commercially in water softening salt compositions, and does remove iron from the system, but citric acid or the mineral acids suggested in some other references can accelerate damage to metal or plastic materials found within a water softener if they are used regularly to recharge the system.
U.S. Pat. Nos. 4,071,446 and 4,116,860, respectively issued to Kunin on Jan. 31, 1978 and Sept. 26, 1978, disclose compositions for regenerating resin beds, comprising a major proportion of an alkali metal chloride, a minor proportion of an alkali metal carbonate, and as the remainder an alkali metal carboxylate chelating agent. Among the many carboxylates disclosed in these references are sodium and potassium citrates. The resins disclosed therein, further characterized in U.S. Pat. No. 4,083,782, are weak acid cation exchange resins adapted to exchange sodium or potassium ions for hydrogen cations, thereby reducing the acidity of the incoming water. No disclosures of the iron problem or the present solution to that problem are made in the Kunin patents. Furthermore, Kunin suggests extremely high concentrations of sodium citrate (5 to 15 per cent of the exemplary compositions).
Oily materials and insoluble particulate matter (which can include precipitated ferric iron) in the water supply also foul cation exchange resins. The particulate matter is bound to the resin beads by the hydrophobic oily matter, and then defies removal when the resin bed is backwashed or otherwise treated with aqueous solutions. The insolubles bound to the resin beads limit the contact area exposed to incoming water, thus fouling the resin bed. U.S. Pat. No. 3,216,932, issued to Heiss et al. on Nov. 9, 1965, discloses a composition consisting predominantly of salt, and containing minor proportions of (1) a dialkali metal sulfonate of an alkylated diphenyl ether; (2) a dialkali metal sulfonate of dinaphthylmethane; and (3) an aqueous mineral acid. The mineral acid can generate fumes which attack metal and plastic components of the water softener, and the surfactant is used at a higher level than is desirable for economic reasons. Heiss also teaches away from the use of sequestering agents. (See column 1, lines 54-63).
Hofheins, "Cleaning Methods for Fouled Cation Exchange Resins", Water Technology, Feb. 1983, pages 21-25, 33, and 41 discusses how to clean resins fouled by various contaminants, particularly iron, by occasional treatment with sodium hydrosulfite, hydrochloric acid, or polyphosphate or organophosphorus sequestering compounds. This article also recommends the removal of fats, oils, and the like by contacting the resin bed with caustic solutions, which are not suitable for household use.
Another class of products, for restoring resin beds which are so fouled that regeneration would be pointless without pretreatment, also employs surfactants. These materials are not adapted for regular use in a water softener. The patents disclosing products of this type include U.S. Pat. No. 3,748,285, issued to Wiltsey on July 24, 1973. Another patent, which discloses the use of a surfactant in a special process for removing entrapped air from a water softener, is U.S. Pat. No. 3,299,617, issued to Dunklin on Jan. 24, 1967. Here again, the composition is not intended to regenerate the resin bed, but is used for a special purpose.
The prior art known to the inventors does not suggest combining a sequestering agent with a surfactant, particularly in the context of a resin regenerating composition for regular use to maintain the cleanliness of the resin, comprising salt, a sequestering agent, and a surfactant.
The art also has not recognized the special characteristics required of a surfactant for use in a salt composition intended for routinely regenerating cation exchange resin beds. Surfactants for this purpose must be anionic, as cationic surfactants would bind to the resin exchange sites and nonionic surfactants are not soluble in brine. The candidate surfactant must be safe for human consumption at low levels in drinking water, as minute quantities of the regenerating composition might be carried into the treated water supply. The surfactant must be low foaming at the normal level of use, to prevent the introduction of air into the softener. The surfactant must be soluble in saturated brine (which few surfactants are). It must be compatible with hard water, so no insoluble precipitates are formed. Finally, the surfactant must be sufficiently inexpensive to be economically justifiable and must be stable at the high temperatures encountered when compacting granulated salt to form products such as pellets.